How can I constrain a bone to follow another bone from the same armature?
Done! It turns out I was trying to add the constrains in the OBJECT CONSTRAINS tab, instead of BONE CONSTRAINS. I'm glad it way my fault because of my lack of experience with Blender.
Sorry for my ignorance and thanks for your help, you were right, you can make a bone child to another bone in the same armature with BONE CONSTRAINS! :)